In forming a thin disk-shaped substrate, such as a CD or a DVD, by an injection molding machine, only a small quantity of a molten resin and only a short molding cycle time of several seconds are used for forming one disk, because the disk is thin. A large quantity of digital information is transferred from a stamper to the disk under such a severe condition.
In forming an optical molding, such as a thin disk-shaped substrate, by injection molding using a resin, internal strain is liable to be generated because of molecular orientation in the resin when the resin is injected into a die to fill up a cavity. Consequently, birefringence and warpage angle (camber angle) are liable to increase. Therefore, the mold is clamped by a low mold clamping force during a filling step to suppress the internal strain that may be caused during the filling step. Generally, an injection compression circuit is used to increase the mold clamping force at the completion of the filling step, or before and/or after the completion of the filling step, and to ensure the formation of recording pits.
An injection compression molding method of forming a thin disk-shaped substrate holds a low mold clamping pressure during an injection and filling step. Therefore, the pressure of a molten resin filled in the mold opens the mold and, consequently, the thickness of a cavity corresponding to the thickness of a disk-shaped substrate increases. Thus, strain caused by a shearing stress induced in the surfaces of a disk-shaped substrate being molded can be suppressed at a low level. The smaller the strain caused by the shearing stress, the smaller are the birefringence and the warp of the substrate, and hence warpage angle is reduced. However, an effort to reduce birefringence and warpage angle and an effort to form accurately recording pits conflict with each other.
FIG. 3 is a diagram showing an instruction waveform indicating levels of mold clamping pressure P for a clamping part, with respect to time t after starting an injection phase, in a conventional method of molding a thin disk-shaped substrate. In FIG. 3, operations (phases) to be carried out by an injection part are shown for reference under the horizontal axis on which the time t is measured. Generally, a mold clamping step to be carried out by the mold clamping part includes a clamping pressure holding phase and a clamping and compressing phase. In the clamping pressure holding phase, a molten resin is filled mainly into a cavity, i.e., a forming space, substantially in cooperation with the filling phase of the injection unit. In the clamping and compressing phase, the formation (transfer) of recording pits or the like is achieved mainly, substantially in cooperation with a pressure holding phase by the injection unit.
As show in FIG. 3, a comparatively low clamping pressure PA is held for a term T in the conventional clamping pressure holding phase. Subsequently, after a time t2, a comparatively high clamping pressure PB is held while the transfer operation is carried out. The clamping and compressing phase is terminated at a time t4 during a cooling phase.
As apparent from the foregoing description, a single level of clamping pressure is held in the conventional clamping pressure holding phase. Therefore, it is difficult to mold a substrate satisfying all the requirements, i.e., a desired degree of birefringence, a desired degree of warpage angle and a desired transfer characteristic. More concretely, although birefringence and warpage angle may be satisfactorily reduced when the clamping pressure is set comparatively low, a desired transfer characteristic cannot be achieved. On the other hand, although a transfer characteristic is satisfactory when the clamping pressure is set comparatively high, birefringence and warpage angle are excessively large.